Underground storage tanks used primarily for the storage of gasoline are currently made of metal or a fibrous reinforced resinous material. The all metal tanks are cylindrical shaped, normally with flat end caps. The fibrous reinforced resinous tanks also are cylindrical shaped, with one tank variation having a ribbed appearance. The ribbed variation has a series of evenly spaced ribs extending circumferentially around the main body of the tank. The ribs significantly strengthen the tank. Additionally, they are able to absorb a certain degree of transport and installation external forces, which normally would have to be absorbed by the main body of the tank.
There are two methods which are widely used for building the ribbed fibrous reinforced resinous tanks. In one method, the main body of the tank is formed over a collapsible cylindrical shaped mold. Thus, a half mold having a smooth surface of proper shape has fibrous material and resinous material applied to its outer surface and cured. The basic wall forming components are applied separately or concurrently from separate spray guns. After proper curing, the mold is collapsed and removed. A second half tank is formed in a similar manner. The two half tanks are joined and secured together. A set of ribs are placed in desired locations on the exterior of the tank and adhered thereto by applications of fibrous and resinous materials.
A second method of building a ribbed fibrous reinforced resinous tank also uses a mold. However, in this method, the mold itself is shaped so that ribs are formed along with the rest of the main body. That is, the molds are provided with the mirror image of ribs. Fibrous material and resinous material are applied as in the first described method and ultimately cured.
Attempts have been made in recent years to build double walled steel and fibrous reinforced resinous tanks. Obvious methods have involved simply forming two tanks, one slightly smalled in diameter and length. The two properly dimensioned tanks are combined, normally with spacers, and used. Ribbed tanks having a double wall construction are not conducive to being formed in this manner.
There is a need for a double walled ribbed storage tank system. Such tank systems would have secondary containment capability because of the double wall construction. Such systems would also have the strength characteristics resulting from the rib feature. The problem has been how to build such a tank system in an efficient manner. There has now been discovered a method of building double walled ribbed storage tank systems using a unique sequence of construction steps. The method is efficient in terms of labor savings and raw material savings. The method is also attractive in that fumes normally given off in any process using resinous materials are confined and thus readily controlled.